In large scale computer systems, it has become necessary to store great amounts of digital data into memory systems which need to have the capability of being randomly accessed. For example, a host computer may control a database system which is accessible by subscribers through remote computer terminals. The database system may for example contain information from diverse arts and disciplines. The database may be divided into general libraries, and each library divided into sublibraries or files. An example of one such library may include publication library wherein each of its files contain in digital form complete transcripts of a selected nationally known newspaper or periodical from the present and extending back in time for a given number of years. Another such library may include digital record of reported court cases wherein each file designates particular courts such as the U.S. Supreme Court or Federal Courts of Appeals. Presently, the size of each of the database files is only limited by the efforts to date of the database owner to compile and digitally translate numerous volumes of printed material. Another example of a database system includes a centralized random access database to support distributed computer aided design, manufacturing (CAD/CAM/CAE) capabilities of a manufacturing entity.
It is also desirable to minimize the overall dimensions of such memory systems. Digital optical disk memory systems have been the most promising for storing large quantities of data because of the large amount of data which can be digitally stored on a single disk. By incorporating many such disks into one memory device, one such memory device may store entire libraries of information.
A digital optical disk having a diameter of approximately 20 centimeters can presently hold around 0.5-0.7 gigabytes of information on one surface thereof. Systems have been developed in the prior art to automatically handle many such optical disks. Known systems may handle anywhere from between tens and hundreds of such optical disks. These systems are often called "juke box" systems and are based upon complex and sophisticated mechanics for performing through automatic loading and unloading of optical disks on one or more optical disk drives. The selection of the optical disk is made by addressing the memory device, which contains address decoders, to activate the mechanics and select the proper disk.
When the optical disk memory system is addressed, a dedicated robot first performs an automatic store or retrieve of a selected disk platter, the selection being indicated by the decoded address. Furthermore, the robot may also perform automatic loading and unloading of the optical disk on a selected one of several disk drives in the system.
One prior art system, known as a carousel juke box system, has an optical disk drive unit and a rotatable carousel for holding the optical disks associated therewith. The optical disks disposed in the carousel and held so that each disk defines the cross section of a toroid. When the disk drive unit is addressed, the toroid of optical disks is rotated so the addressed disk is placed into a selected position. A retrieval arm associated with the disk drive unit grabs the disk and places it on the disk drive unit.
Another prior art system is known as a linear juke box system. In the linear juke box system, a plurality of optical disks are stored face-to-face in one or more stacks. When addressed, a retrieval system will move adjacent to the proper stack and a retrieval arm thereof which then pull the addressed disk from the stack. The retrieval system then moves to place the optical disk held by the retrieval arm on a selected or free disk drive in the system. Such a system may have a plurality of optical disk drives so that retrieval and storage of one disk may be made while another disk is presently on a disk drive.
However, when the capacity of the optical disk memory system must be in the order of tens of gigabytes or greater and the random access time must be kept as low as possible, and very often in less than one second, the carousel and linear juke box system exhibit major disadvantages and limitations. One such disadvantage and limitation is that the access time is relatively slow due to the mechanical transportation of the disk within the system. Another disadvantage and limitation is that the system is vulnerable to mechanical failure. For example, a failure in the retrieval and storage mechanics of the memory system will place a large number of optical disks out of access. Furthermore, the very sophisticated disk handling mechanisms in such mass storage devices do not provide for a real time update of disk libraries. This is because to load and unload optical disks into and from the system becomes a slow procedure and very difficult to implement safe backup procedures.